Sustainable two-stage thermochemical conversion of sewage sludge for the co-production of biochar and carbon nanotubes

The increasing volume of sewage sludge, along with its significant environmental burden and disposal challenges, necessitate sustainable waste management solutions. This study investigates the potential of co-producing biochar and carbon nanotubes (CNTs) from sewage sludge through a two-stage process integrating pyrolysis and catalytic chemical vapor deposition (CCVD), presenting an innovative sustainable approach to waste valorization. In the first stage, sewage sludge underwent slow pyrolysis at 550°C, yielding 66 wt.% biochar. The produced biochar exhibited a B ET surface area of 24.40 m²/g and elemental composition of C (20.9%), H (0.4%), N (2.4%), S (1.7%) and O (3.3%). The organic vapor generated from sewage sludge pyrolysis was then introduced into a secondary reactor for CCVD, where a cobalt catalyst facilitated the formation of CNTs. Process parameters, including temperature (650 to 950°C) and catalyst loading (0.2 to 0.8 g), were optimized using response surface methodology (RSM) coupled with a central composite design (CCD) model.